Electromechanical transducers are machines, which convert electrical energy into mechanical energy or vice versa. An electric motor is a widely used electromechanical transducer that converts electrical energy into mechanical energy using magnetic field linkage. An electric generator is an electromechanical transducer that converts mechanical energy into electrical energy also using a magnetic field linkage.
An electromechanical transducer comprises a stator and a rotor. The stator is an assembly, which represents the stationary part of an electromechanical transducer. The rotor is an assembly, which represents the moving and in particular the rotating part of an electromechanical transducer.
In order to provide magnetic field linkage, permanent magnets may be used in particular for a rotor of an electromechanical transducer. In recent years, especially since the introduction of rare-earth magnetic materials, permanent magnet (PM) electromechanical transducers have become popular since they eliminate the need for commutators/collectors and brushes, which are commonly used with conventional Direct Current (DC) electromechanical transducer. The absence of an external electrical rotor excitation eliminates losses on the rotor and makes permanent magnet electromechanical transducers more efficient. Further, the brushless design of a PM electromechanical transducer allows conductor coils to be located exclusively in the stationary stator. In this respect, it is mentioned that non-PM electromechanical transducers, which are equipped with commutators and brushes, are typically susceptible to significantly higher maintenance costs.
PM-electromechanical transducers are also known for their durability, controllability and absence of electrical sparking. PM-electromechanical transducers are widely used in many applications such as electric vehicles (electromechanical transducer is a motor) or in power generation systems (electromechanical transducer is a generator) such as for instance a wind turbine.
One challenge of PM-electromechanical transducers is cogging torque. Cogging torque is produced between the rotor and the stator due to a spatial meeting of “sharp edges” of rotor mounted permanent magnets and stator coils when the PM-electromechanical transducer is in operation. In a PM-electric generator, cogging torque is an undesired effect that contributes to a so called “torque ripple” on the electric power output signal of the generator.
Another challenge of PM-transducers is the mechanical “radial force/pressure ripple”. Both effects lead to unwanted vibrations of the PM-electric generator, which cause troublesome acoustic noise. This holds in particular for PM-electric generators, which in particular in wind turbine applications are operated with variable rotational speed. Avoiding or at least reducing acoustic noise and vibrations is a great challenge in the design and/or the operational control of PM-electric generators being used for wind turbines.
One approach to reduce acoustic noise and vibrations is to choose an appropriate design for a PM-electric generator. For example, torque ripple can be reduced by using an appropriate shaping of magnets. Further, a skewing of the rotor magnets has been proposed (e.g. U.S. Pat. No. 6,867,524 B2). However, it is difficult to minimize both torque ripple and radial force/pressure ripple by using a single design for a PM-electromechanical transducer.
Another approach to reduce noise and vibration within a PM-electromechanical transducer is to design mechanical parts of the transducer in such a manner that all resonance frequencies, which can be excited by torque and radial force/pressure ripple, are out of the frequency range. However, in case of a variable rotational speed of the PM-electromechanical transducer, the frequency range that can be excited by torque ripple and/or radial force/pressure ripple is very wide. Therefore, it is very difficult to avoid vibrations within the whole frequency range of the PM electromechanical transducer.